Math: Use C99 fpclassify

Currently checks #if defined(PETSC_HAVE_C99) and swaps out isnan(), isnormal(), isinf() for fpclassify(). Keeps quadmath functions intact.

Should we require C99 support all of the bogus

PetscBool PetscIsXXXXXReal(PetscReal a)
{
  return PETSC_TRUE;
}

Can be gotten rid of

cc @jedbrown